Delete any existing licenses.html before transforming to create a new one

If the transform task in the tasks.xml from the docs.licenses.merge galleon package from more than one f-p gets executed it will fail because the first task to run will create a licenses.html and 'transform' will fail if the target file already exists. So as a safety measure have the tasks delete any such file before doing the transform.

AIUI a fix for GAL-305 will remove the need for this by preventing more than one docs.licenses.merge package being provisioned, but having this in there will do no harm and makes things a bit more robust.

Also, remove the crufty 'alternative-tasks.xml' files from these docs.licenses.merge packages.